This article is more than 5 years old. Information may no longer be current.
MIC is only part of overall effectiveness
Click Here to Manage Email Alerts
[MAIN ARTICLE: The anti-infective equation: finding the right drug mix to fight infectious organisms]
[RELATED ARTICLE: When do I culture?]
Attempts to prevent and control infection can be challenging and costly to practitioners and patients. Clinicians must quantify the effectiveness of anti-infective agents on different organisms to help determine the clinical outcomes of the chosen therapy.
Both the minimal inhibitory concentration (MIC) and inhibitory quotient (IQ) aid practitioners in evaluating and choosing appropriate therapies. A standardized method to measure penetration and potency, however, has yet to be established. When achieved, the standardized method also would be used for drug evaluation.
The ultimate ophthalmic anti-infective would achieve sufficient sustained concentration to kill bacteria in the eye; have a half-life above the MIC of the microorganisms; be nontoxic to the endothelium, trabecular meshwork and retina; and be effective against the organism without the risk of developing resistance.
"Antimicrobials can be compared by their MICs, that is the amount of each drug it takes to inhibit an organism," says Clyde Thornsberry, PhD, director of MRL Pharmaceutical Services, Franklin, Tenn. "Such a comparison may not be clinically useful, however, unless the concentration of drug at the site of infection is known."
For clinical purposes, an IQ — defined by the site concentration divided by the MIC of the drug — would best be done using the MIC of the infecting organism and the concentration of the drug in that particular patient. However, the amount of medicine in the specific patient is rarely known, Dr. Thornsberry says.
MIC90, concentrations that inhibit 90% of the strains in a species and published concentrations of the antibiotic for the infected tissue, can be replaced with MIC for customary purposes and comparisons.
According to Dr. Thornsberry, the use of the IQ is suitable for comparing drugs used for treating patients with ocular infections. The IQ takes both the potency and penetration values and represents both by one value.
James P. McCulley, MD, in consultative practice and a professor and chairman at the University of Texas-Southwestern in Dallas, initially believed the IQ would be unmanageable. "However, I began to accept it because I disagreed with those pushing the idea that penetration was the only important aspect in the comparison of drugs, " Dr. McCulley says. "The IQ is useful for two reasons: it allows us to take both penetration and potency into consideration, and it allows us to use one simple number to represent those two important factors meaningfully."
"The IQ links the pharmacokinetic properties with the microbiologic efficacy in a dynamic way that avoids the artificial separation of these two equally important parameters in treating eye infections," says Terrence O’Brien, MD. Although the IQ will not replace the need to establish an authentic ocular MIC, it does, according to Dr. O’Brien, assist in compound evaluation when selecting medicines for patients.
For Your Information:
- James P. McCulley, MD, can be reached at 5323 Harry Hines Blvd., Dallas, TX 75235-9057; fax: (214) 648-9061.
- Terrence P. O’Brien, MD, can be reached at Wilmer Eye Institute, 600 N. Wolfe St. Wilmer Woods 255, Baltimore, MD 21287-9121; (410) 955-1671; fax: (410) 614-0682.
- Neither Dr. McCulley nor Dr. O’Brien have a direct financial interest in any product mentioned in this article, nor is either a paid consultant for any company mentioned. Clyde Thornsberry, PhD, can be reached at 357 Riverside Dr., Franklin, Tenn. 37064; (615) 794-2244; fax (615) 794-7705. Dr. Thornsberry did not disclose whether he has a financial interest in any product mentioned, nor if he is a paid consultant for any company mentioned.
- Reference: Ophthalmic Fluoroquinolones, supplement to the Aug. 1, 1997, issue of Ocular Surgery News.